Photovoltaic cells convert light energy (e.g., sunlight) into electricity for a variety of applications. Often used in solar cells, the power production from photovoltaic cells offers a number of advantages such as low operating costs, high reliability, modularity, low construction costs, and of course environmental benefits.
Solar cells convert light into electricity by exploiting the photovoltaic effect that exists at semiconductor junctions. Accordingly, solar cells generally implement semiconductor layers to produce electron current. The semiconductor layers absorb incoming light to produce excited electrons. In addition to the semiconductor layers, solar cells include a front contact electrode to allow the electrons to enter a circuit, and a back contact electrode to allow the ions created by the excitation of the electrons to complete the circuit.
Solar cells based on organic materials have drawn considerable interest in recent times due to the opportunity to decrease cost based on the fact that polymers are cheaper to produce than inorganic semiconductors. Cost per kWhr has been the major market deterrent for widespread use of solar power. Cost can be reduced by using organic-based materials; however, organic-based solar cells are not as efficient as their silicon counterparts. One of the main reasons is due to the types of materials employed in organic photovoltaics (OPVs) and the ability of these materials to effectively generate and transfer electrons and holes (i.e., charge carriers opposite of electrons). The materials that absorb light and transfer electrons (active layers) have been implemented as blends of different materials (bulk heterojunction), stacks of different materials, or adsorbed or chemically attached to inorganic semiconductors (dye-sensitized solar cells). These active layers of OPVs comprise organic-based materials that act as electron donors (donors) or electron acceptors (acceptors). Generally, these donors and acceptors have different dielectric constants, RUB ratios, functional groups, or other property that affects the electrical properties of the materials. Current bulk heterojunction OPVs suffer from phase segregation in the active polymer layers due to mismatch between the donors and acceptors. This creates alternative paths for electrons within the active layer of the solar cells that sacrifice the power output of the cell and diminish the efficiency of the cell. The optimal organic active layer should comprise a homogenous material with donors and acceptors in a nearest neighbor configuration that increases the probability of electron transfer between the donors and acceptors.
Ionic liquids (ILs) play an important role in many electrical devices, including dye-sensitized solar cells (DSSCs), organic light emitting diodes (OLEDs), batteries, and supercapacitors. Currently, ILs are used to transport charge in these devices based on diffusion of ions from one electrode to another through the charge transfer medium known as the electrolyte. This type of use has been applied to solar cells, and in particular DSSCs. These cells use an organic dye to absorb incoming light to produce excited electrons. The DSSC likewise includes two conducting electrodes arranged in a sandwich configuration with a dye-coated inorganic semiconductor film separating the two electrodes. One exemplary technique for fabricating a dye-sensitized solar cell is to coat a conductive glass plate with a semiconductor film such as titanium oxide (TiO2) or zinc oxide (ZnO). The semiconductor layer is usually porous and has a high surface area to allow sufficient amounts of dye for efficient light absorption to be attached as a molecular monolayer on its surface. The semiconductor film is then saturated with a dye and a single layer of dye molecules self-assembles on each of the particles of the semiconductor film, thereby “sensitizing” the film. The remaining intervening space between the electrodes and the pores of the semiconductor film is filled with an electrolyte solution, which may be or may contain a conventional ionic liquid. The electrolyte fills the pores and openings left in the dye-sensitized semiconductor film. The purpose of the electrolyte is to transport the charge generated by the light-excited dye to the electrodes. To complete the solar cell, another electrode is used to provide a cell structure having the dye-sensitized semiconductor and electrolyte sandwiched between the electrode and the conductive glass plate.
It is clear that despite the recent and various advances in photovoltaic materials and devices that contain them, new photoactive materials are still needed. For example, what are needed are materials that absorb within the visible range of the electromagnetic spectrum, can avoid deleterious phase segregation that hampers bulk heterojunction OPVs, are lightweight, flexible, easy to use, and are compatible with fabrication techniques such as spraying, screen-printing, and roll-to-roll processing. The subject matter disclosed herein addresses these and other needs.